Soft-Tissue Plasmacytomas in Multiple Myeloma: Incidence, Mechanisms of Extramedullary Spread, and Treatment Approach

We provide an overview on soft-tissue extramedullary plasmacytomas (EMPs) in multiple myeloma (MM). We reviewed the incidence of EMPs in MM, myeloma bone marrow homing, possible mechanisms of extramedullary spread, and prognosis and response to therapy. The incidence of EMPs is 7% to 18% at MM diagnosis and up to 20% at relapse. The current notion that EMPs are more frequent after treatment with novel agents remains to be proven, especially considering that different patterns of disease recurrence can emerge as patients live longer in the era of novel drugs. Bone marrow genetic abnormalities are not associated with extramedullary spread per se, which also suggests that microenvironmental interactions are key. Possible mechanisms of extramedullary spread include decreased adhesion molecule expression and downregulation of chemokine receptors. EMPs usually show plasmablastic morphology with negative CD56 expression. High-dose therapy with autologous stem-cell transplantation (ASCT) can overcome the negative prognostic impact of extramedullary disease in younger selected patients. EMPs do not typically respond to thalidomide alone, but in contrast, responses to bortezomib have been reported. The incidence of EMPs in patients with MM is high and is associated with poor outcome in patients treated conventionally. A potential first-line treatment option seems to be a bortezomib-containing regimen followed by ASCT, whenever possible. Experimental studies on the mechanisms of myeloma cell adhesion, myeloma growth at extramedullary sites, and drug sensitivity are priorities for this area of continuing therapeutic challenge.